Civ: Beyond Earth's Rising Tide expansion might not have done enough to dredge Firaxis' game up from the swamps of mediocrity, but it did feature a horrible bug that kept the diplomacy system from working as it was supposed to. Oh, wait, that's bad. The bug made it so that your allies sometimes wouldn't join in when you've decided to do a war, and that's the absolute height of rudeness in my opinion.
Anyway, it's a moot point now, as Firaxis has issued a quick hotfix that eliminates the bug.
Here's Firaxis' David Hinkle explaining the problem, and how it's been fixed:
"Previously, there were some cases where a player would go to war, but their allies would not join them in the fight. Now, when a player declares war, all the player’s allies will also declare war on the target. If you are at war, and then you form an alliance with a faction that has not yet met your opponent, your new ally will not join the war automatically, since they don’t have prior diplomatic contact. This change will not apply to existing save games with an active war or alliance, but will apply to all games going forward."
As patches are wont to do these days, this one will automatically download itself on Steam. In fact, it probably already has, but I thought I'd tell you in case you were wondering why diplomacy was suddenly working.